Context Navigation

From Luc Frauciel, "I've done 2 main modifications :
1) added texture->setResizeNonPowerOfTwoHint(false); when loading an
image. It speeds up by 10 the loading of large images.
2) added a --disk option : only a filelist is read, images are only
loaded when needed. It allows to handle very large set of very large
images that would not fit in memory. Nothing change when the option is
not set."